There exists in medicine an important and continuing need to be able to determine the velocity of a fluid under investigation, particularly blood, which flows through a vessel or through connected instrumentation. Conventional ultrasonic transducers employing Doppler measurement techniques are commonly used for this purpose. See Atkinson and Woodcock, Doppler Ultrasound and Its Use In Clinical Measurement, Academic Press, London (1982). "Doppler" is used here in the broad sense to describe all the techniques for measuring the variation with time of backscattered ultrasound along a beam to determine the velocity component along the beam. These methods include what is conventionally considered Doppler--i.e., the change in phase of backscattered ultrasound as a result of movement--as well as newer time-domain methods that use cross-correlation to determine the velocity along the beam, See Hoeks et al, "Comparison of the Performance of the RF Cross-Correlation and Doppler Auto-Correlation Technique to Establish the Mean Velocity", in Ultrasound in Medicine and Biology 19, page 727 (1993). All such methods measure the velocity component along the beam. To determine true fluid velocity, which is the subject of this invention, requires the correction for angle between the beam and the velocity vector. The results for cross-correlation methods, as well as for the Doppler methods, will depend upon the frequency in use. As will be shown, the angle of the beam from the transducer structure disclosed will also vary with the frequency used. For all such methods, one can obtain two equations, relating the measured effects of changing frequency in terms of the observed velocity and angle, and then determine both velocity and angle between the measuring ultrasound beam and the velocity vector. For ease of use herein, reference to the term "Doppler" will signify reference to all methods that use time variation of the backscattered ultrasound along a beam to measure the velocity of a fluid at an unknown angle to the beam.
Ultrasonic transducers are devices which convert energy between electrical and acoustic forms. See L. Kinsler et al., Fundamentals of Acoustics, John Wiley & Sons (3d ed. 1982). By directing an insonifying beam of ultrasonic energy towards a fluid under investigation at a known angle, and by then measuring the frequency shift of the backscattered ultrasound energy, the velocity of the fluid under investigation can be determined. This is because the Doppler shift in frequency is proportional to the component of the velocity vector that is parallel to the insonifying beam. The well-known equation for finding the velocity v of the fluid from the Doppler shift frequency f.sub.d is ##EQU1## where c is the velocity of sound in blood, f is the frequency of the insonifying beam and .theta. is the angle between the insonifying beam and velocity vector.
A problem commonly encountered when employing conventional Doppler techniques to measure the velocity v of a fluid under investigation is that typically the insonifying beam insonifies the fluid flow at an unknown angle. Without knowledge of the angle, equation (1) cannot be solved. Therefore, in such situations, employing a single insonifying beam to determine the velocity of the fluid of interest from equation (1) is impossible since there are two unknowns in the single equation (1), the velocity v and the angle .theta..
One method of eliminating the foregoing problem of determining the angle .theta. in question has been to employ two transducers at a known angular offset of .+-..alpha. and to insonify the fluid under investigation at the respective angles of .theta.+.alpha. and .theta.-.alpha., thus allowing the two following equations (2) in the two unknowns v and .theta. to be written, and by solution of two equations in two unknowns, permitting v to be determined regardless of the value of .theta.: ##EQU2## see Overbeck et al, "Vector Doppler: Accurate Measurement of Blood Velocity in Two Dimensions", Ultrasound in Medicine and Biology, Volume 18, page 19 [1992]). However, using two conventional transducers at two angles to the fluid flow is difficult, as transducers are thick, inflexible, bulky, difficult to implant, and too big to be useful on a catheter or a guidewire. Thus, use of pairs of transducers, while possible, is rare.
Conventional phased-array ultrasound transducers used for diagnostic imaging are all essentially configured as an array of linear elements. A major problem with these conventional transducers is their complexity--as shown in FIG. 1, each of the linear transducers that compose the array has its own connection and driver. These are necessary for the phase adjustment needed to focus and steer the beam the array produces. In the case of what is known in the art as a linear array, only certain groups of elements at a time are connected in parallel; these elements act as one uniform transducer and produce one beam, which is scanned by changing which group of array elements are connected. Operationally, conventional phased array and linear array transducers suffer the drawback of requiring many signal cables to operate, either to connect to each element or to connect to groups of elements.
Most conventional phased array ultrasound transducers operate by generating a single beam which is scanned over an angular sector of beam positions. Reflections are obtained at a multitude of beam locations to determine velocity of a fluid being targeted. Other imaging systems are presently available which operate by generating multiple beams from a phased array transducer, by using superposition of the driving patterns for each beam. For example, U.S. Pat. No. 5,105,814 discloses a method of transforming a multi-beam ultrasonic image in which a plurality of ultrasonic beams are simultaneously transmitted into an object. This method does not employ a Doppler measurement technique to arrive at a velocity of a fluid. Moreover, these multibeam systems also suffer from the problem of requiring a multitude of transducer elements, phase delays, and cables, resulting in a complex and bulky configuration.
Diffraction is well-known in optics as a way to produce multiple optical beams based on the interference effects of waves. Likewise, it is conceivable to use diffraction principles in ultrasound as a way to produce multiple ultrasonic beams for performing Doppler measurements of the velocity of blood or other fluids. The conventional phased-array ultrasound transducer configuration of an array of linear elements may appear similar to the appearance of a diffracting structure (of which the present invention relates to). However, these conventional transducers operate based on a totally different principle as compared to the diffraction principle of the present invention.
It is therefore an object of the present invention to provide a single transducer that is able to produce multiple insonifying beams at known and controllable angles by employing diffraction principles, thus permitting the absolute velocity of a fluid under investigation to be determined using Doppler measurement techniques.
It is a further object of the present invention to provide such a transducer which is thin, flexible and easy to implant, and which eliminates the need for multiple cables and elements as required by the prior art.